Method of forming via holes in an insulation film and method of cutting the insulation film

Abstract: A method of forming via holes (6) in an organic insulation film (1) or cutting the film (1) comprises the steps of exposing predetermined parts of the film (1) to a laser beam to raise a temperature of the exposed parts of the film (1) until the exposed parts are transformed or decomposed and subjecting the film (1) to an ultra sonic wave (5) so that the transformed or decomposed parts are dispersed.

Description: The invention relates to a method of forming via holes in an insulation film including organic polymers to be used as a substrate for high density multilevel interconnections as well as a method of cutting the insulation film in various desired patterns. A method for cleaning blind holes created by laser drilling in a printed circuit is known from IBM Technical Disclosure Bulletin, Vol. 20, Nr. 3, August 1977 "Ultrasonic Chemical Method of Cleaning Blind Holes in a Printed Circuit Board". According to this method, residual material in the blind holes is removed by subjecting the printed circuit, e.g. made of epoxy glass, to an ultrasonic wave. A method for forming a pattern of a film on a glass substrate with a transparent conductive film, e.g. made of SnO 2 to be processed. with a laser beam is known from US-A-47 86 358. The multilevel interconnection techniques may permit an improvement of a high density packaging of large scale integrated circuits for computers. One of the important issues on the integrate circuit packaging techniques is to reduce the manufacturing cost as well as to place the high density packaging technique in a practical use. The insulation film used for the high density packaging of the integrated circuits are required to be made of such materials as to permit the integrated circuits to show high frequency performances. Teflon resin, polyimide resin and glass epoxy resin are available for the insulation film material wherein the insulation film material may include silica. In a further prior art, the via holes are formed in a glass epoxy resin substrate by use of mechanical drills. Further it has been known in the art that to form the via holes in the organic film photo-etching of polymer films by far-ultraviolet excimer laser radiation is available, which is disclosed in Applied Physics Letter 41(6) 15, September 1982, pp. 576-578. In the prior art for cutting the glass epoxy resin substrate, is has been known to use various mechanical cutter. To cut the organic films, it has been known to use carbon dioxide gas laser radiations to cause a melting of the organic film and subsequent evaporation thereof. The above method of forming the via hole by use of the mechanical drill has disadvantage as described below. A time of 0.1 seconds is needed to form a single via hole in the glass epoxy resin substrate. As a result, a formation of 2000 of via holes in the 20cm-squared substrate requires about 3 minutes. This provides a limitation in throughput thereby resulting in a difficulty in reduction of the manufacturing cost. When forming the via hole in the Teflon film suitable for high frequency performance of the device but having poor thermal and mechanical properties, it is required that a rotation speed and a feeding speed of the drill are slow. In use of the mechanical drill, a minimum diameter of the via hole is relatively large for example 200 micrometers. The above method of use of the photo-etching of polymer films by far-ultraviolet excimer laser radiation has also disadvantages as described below. This method may permit forming a fine via hole having a diameter of a few micrometers. Notwithstanding, in use of this laser radiation, a maximum processing speed is 0.1 micrometers/pulse. When use of the commercially available excimer laser of 100 Hz, about 2 seconds are needed to form only a single via hole in a thin insulation film having a thickness of 20 micrometers. Even if one shot laser radiation may form a plurality of via holes, then the processing speed to form the via hole in the 20cm-squared printed board is 1/5 times of the processing speed in use of the mechanical drill thereby resulting in a difficulty in improvement in the throughput. Further, this laser radiation method is available to only a film that shows a high absorption property of the laser beam. The ExCl excimer laser or KrF excimer laser are not available to the Glass epoxy resin film and the Teflon resin film. The excimer laser uses halogen gases such as fluorine or chlorine. This results in a high cost of the laser device or a high maintenance cost. The above conventional method for cutting the film by use of the mechanical cutter also has a disadvantage in that the cutting edge of the film has a flash and a chamber. Further, a cutoff area of 300 micrometers is needed. The use of the carbon dioxide gas laser to cut the film also has a disadvantage in that a thermal transformation caused by the laser radiation remains at the cutting edge of the film because the cutting edge is subjected to a higher temperature than a melting point of the film. Accordingly, it is an object of the present invention to provide a novel method of forming via holes in an organic insulation film comprising a base material of a Teflon resin added with a filler material of glass particles or compound particles, free from any disadvantages as described above. It is another object of the present invention to provide a novel method of cutting an organic insulation film film comprising a base material of a Teflon resin added with a filler material of glass particles or compound particles, free from any disadvantages as described above. The above and other objects, features and advantages of the present invention, as defined in claims 1 and 3 will be apparent from the following descriptions. Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. FIGS. 1A to 1C are cross sectional elevation views illustrative of an insulation organic film in sequential steps involved in a method of forming via holes in the organic insulation film. FIG. 2 is a plane view illustrative of an organic insulation film with via holes. FIGS. 3A and 3B are cross sectional elevation views illustrative of an insulation organic film in sequential steps involved in a method of forming via holes in the organic insulation film. FIGS. 4A to 4C are cross sectional elevation views illustrative of an insulation organic film in sequential steps involved in a method of cutting the film. FIG. 5 is a schematic plane view illustrative of an organic insulation film with via holes. FIGS. 6A and 6B are cross sectional elevation views illustrative of an insulation organic film in sequential steps involved in a method of cutting the organic insulation film. The invention provides a novel method of forming via holes in an organic insulation film wherein predetermined parts of the organic insulation film are selectively exposed to a laser radiation to cause a thermal transformation or a thermal decomposition of the exposed parts thereof for subsequent ultrasonic cleaning of the film. The organic insulation film comprises a base material made of Teflon resin added with a filler material of glass particles or compound particles. The laser beam is irradiated on a predetermined part of the organic insulation film to cause a transformation of the predetermined part of the organic insulation film or to cause a thermal decomposition thereof. Subsequently, the ultra sonic cleaning of the film is carried out. Since the absorptivity of the organic insulation film is not high, then it is provided an absorption film having a high absorptivity to the laser beam on the organic insulation film so that the laser beam is sufficiently absorbed by the absorption film to cause the transformation or the thermal decomposition of the predetermined part of the organic insulation film. The absorption film has a thickness of 0.1 micrometers. A weak laser beam radiation on the absorption film is carried out for a short time of 1 microsecond to 100 milliseconds to raise a temperature of the absorption film thereby the transformation or the thermal decomposition is caused at part of the organic insulation film under the exposed part of the absorption film. The organic insulation film is subjected to the ultra sonic cleaning to cause dispersions in a liquid of the base material of the organic insulation film and the filler material to thereby form a via hole in the organic insulation film. Immediately after the laser beam radiation, the film was observed by an electron microscope so that it was confirmed that the absorption film is lost while the organic insulation film remains as a film with a homology being somewhat varied. As a result, it was appreciated that the laser beam radiation may render an atomic bonding weak between the base material and the filler material thereby the transformation or the thermal decomposition is caused in the organic insulation film wherein the transformed part thereof has a fine definition. The transformed part of the organic insulation film is subjected to the strong ultra sonic vibration to be dispersed to thereby form a via hole having a fine definition. The invention also provides a novel method of cutting an organic insulation film wherein predetermined parts of the organic insulation film are selectively exposed to a laser radiation to cause a thermal transformation or a thermal decomposition of the exposed parts thereof for subsequent ultrasonic cleaning of the film. The laser beam is irradiated on a predetermined part of the organic insulation film to cause a transformation of the predetermined part of the organic insulation film or to cause a thermal decomposition thereof. Subsequently, the ultra sonic cleaning of the film is carried out to cut the organic insulation film. Since the absorptivity of the organic insulation film is not high, then it is provided an absorption film having a high absorptivity to the laser beam on the organic insulation film so that the laser beam is sufficiently absorbed by the absorption film to cause the transformation or the thermal decomposition of the predetermined part of the organic insulation film. The absorption film has a thickness of 0.1 micrometers. A weak laser beam radiation on the absorption film is carried out at a scanning speed of 1 cm/s to raise a temperature of the absorption film thereby the transformation or the thermal decomposition is caused at part of the organic insulation film under the exposed part of the absorption film. The organic insulation film is subjected to the ultra sonic cleaning to cause dispersions in a liquid of the base material of the organic insulation film and the filler material to thereby cut the organic insulation film. Immediately after the laser beam radiation, the film was observed by an electron microscope so that it was confirmed that the absorption film is lost while the organic insulation film remains as a film with a homology being somewhat varied. As a result, it was appreciated that the laser beam radiation may render an atomic bonding week between the base material and the filler material thereby the transformation or the thermal decomposition is caused in the organic insulation film wherein the transformed part thereof has a fine definition. The transformed part of the organic insulation film is subjected to the strong ultra sonic vibration to be dispersed to thereby cut the organic insulation film with a fine vertical cutting sections but without any flash or curvature. The following descriptions will focus on preferred embodiments of the present invention. A first illustrative example, not falling within the scope of the present invention will be described with reference to FIGS. 1A to 1C wherein a method of forming via holes in an organic insulation film is shown. With reference to FIG. 1A, a Teflon film 1 is prepared so that a laser beam absorption film 2 having a high absorptivity to the laser beam is formed on an entire surface of the Teflon film 1. The Teflon film has a thickness of 20 micrometers and includes silica glass fin particles as a filler material. The laser beam absorption film 2 comprises a gold thin film having a thickness of 300 angstroms wherein 300 angstroms is the absorption wavelength of Ar laser beam so that almost all of the Ar laser beam is absorbed by the gold thin absorption film having the 300 angstroms thickness except for a reflected part of the Ar laser beam by a surface of the gold film. The gold thin film 2 as the laser beam absorption film may be formed on the Teflon film 1 by sputtering. With reference to FIG. 1B, an Ar laser beam radiation apparatus is used. The laser beam radiation apparatus comprises an optical scanner for scanning of the Ar laser beam and a movable stage on which the Teflon film is placed. The movable stage may move both in X and Y directions both of which are vertical to each other. The laser beam is generated by a laser source to be transmitted through a variable attenuator, an ultra sonic tuner, an optical scanner and a condenser lens so that a laser beam 3 having a laser intensity of 30 kW/cm 2 and a beam diameter of 8 micrometers defined by a value of 1/e 2 of beam intensity is irradiated on a surface of the gold laser beam absorption film 2. The optical scanner may show a scanning in an area of 5 cm X 5 cm. The movable stage may permit an enlargement of the laser beam scanning area. At each point, the Ar laser beam is irradiated for 1 millisecond. The irradiation of the laser beam 3 may cause a transformation layer 4 in the Teflon film 1. By use of the above apparatus, 10000 via holes are formed in 10 cm square in 2 minutes only including alignment processes. With reference to FIG. 1C, the Teflon film is immersed into a water in an ultra sonic generator with an output of 100 W and a frequency 100 kHz so subsequent ultra sonic cleaning for 5 minutes. Ultra sonic wave 5 is propagated through the water in the ultra sonic generator onto the Teflon film 1 with the transformation layer 4. As a result, the transformation layer 4 is dispersed into the water to form a via hole 6. The via hole 6 formed in the Teflon film 1 has a fine vertical edge defined by a diameter of 20 micrometers and a depth of 20 micrometers. The above novel method may permit a superior control of the definition of the via hole. Under the constant time for the laser beam radiation, the intensity of the laser beam results in the fact that the diameter of the via hole tends to be large. Namely, the control of the intensity of the laser beam with the constant time for the laser beam radiation may permit control of the diameter of the via hole. It is available to vary the time for the laser beam radiation. If the intensity of the laser beam is increased up to 100 kW/cm 2 , then the necessary time for the laser beam radiation at each point is reduced to 10 microseconds. If the intensity of the laser beam is reduced down to 10 kW/cm 2 , then the necessary time for the laser beam radiation at each point is increased up to 100 milliseconds. The intensity and the time for the laser beam radiation are variable to match the conditions of the thickness of the film and the required throughput. The ultra sonic cleaning process may be carried out to subject a plurality of the Teflon films 1 with the transformed layers 4 to the ultra sonic wave 5 for simultaneous processes of the plural Teflon films 1 to improve the throughput. Although in the foregoing example the laser beam absorption film 2 comprises the gold film, other materials having a high absorptivity to the laser beam such as metal films, insulation films and semiconductor films as well as carbon based film are available. As the metal film, a titanium film, a nickel film and a copper film are available. As the insulation film, a titanium nitride film is available. As the semiconductor film, an alpha-silicon film is available. A vacuum evaporation method, a sputtering method and a chemical vapor deposition method are also available to deposit the laser beam absorption film on the Teflon film 1. According to the invention, in place of the deposition of the absorption film, it is provided that the Teflon film is exposed to an etchant of sodium naphthalene to thereby form an absorption layer mainly comprising carbon and having a thickness of 0.1 micrometers. The absorption layer has a brown color. In place of the Ar laser beam, Kr laser beam, Nd:YAG laser beam and second higher harmonic waves are available. Further, semiconductor lasers such as near infrared ray or visible rays are available. The use of the semiconductor laser may provide advantages in a reduced cost, a small scale and a facilitation of tuning of a laser output. This advantages may further permit use of a plurality of the semiconductor lasers so that a plurality of via holes may be formed in a one time laser beam radiation to improve the throughput. A second illustrative example that does not fall within the scope of the present invention will be described with reference to FIGS. 3A to 3B in which another method for forming via holes in an organic insulation film having a relatively large absorptivity to a laser beam is shown. The laser beam is irradiated directly on the organic insulation film without through any laser beam absorption film. The organic insulation film 7 includes a base material of a photosensitive epoxy resin and a filler material of a mixture of calcium carbonate, barium sulfate and glass particles. The organic insulation film 7 has a thickness of 70 micrometers. An Ar laser having a wavelength of 515 nanometers is used wherein ab absorption thickness is approximately 50 micrometers at which the intensity of the laser beam of 515 nanometers wavelength is reduced to 1/e. As illustrated in FIG. 3A, to form the transformed layer 4 in the insulation film 7, a laser beam radiation is carried out at a radiation power of 1W for 6 milliseconds and at a beam diameter of 6 micrometers. As illustrated in FIG. 3B, the ultra sonic cleaning process for the insulation film 7 with the transformed layer 4 is carried out at the same conditions as the first embodiment to thereby via holes are formed on the insulation film 7 wherein each via hole has a diameter of 10 micrometers and a depth of 70 micrometers as well as a fine vertical edge. The irradiation of the laser beam having a wavelength of the same as or near to the absorption wavelength of the insulation film may provide an advantage in a facility of forming a via hole with a large aspect ratio or the via hole having a large depth as compared to the diameter. The increase of the diameter of the via hole may be obtained by increase of any of the intensity of the laser beam, the time for the laser beam radiation and the laser beam diameter. If the laser beam intensity is increased up to 2W but without any change of other conditions, then a via hole having a diameter of 80 micrometers. For irradiation of the laser beam having a wavelength of the same as or near to the absorption wavelength of the insulation film to form the via holes of the large aspect ratio, it is available to vary the laser beam wavelength, the base material of the film or to add the photosensitive material or a filler material with a high absorptivity to the laser beam. In place of the Ar laser beam, Kr laser beam, Nd:YAG laser beam and second higher harmonic waves are available. Further, semiconductor lasers such as near infrared ray or visible rays are available. The use of the semiconductor laser may provide advantages in a reduced cost, a small scale and a facilitation of tuning of a laser output. This advantages may further permit use of a plurality of the semiconductor lasers so that a plurality of via holes may be formed in a one time laser beam radiation to improve the throughput. A third illustrative example that does not fall within the scope of the present invention will be described with reference to FIGS. 4A to 4C wherein a method of cutting an organic insulation film is shown. With reference to FIG. 4A, a Teflon film 1 is prepared so that a laser beam absorption film 2 having a high absorptivity to the laser beam is formed on an entire surface of the Teflon film 1. The Teflon film has a thickness of 20 micrometers and includes silica glass fin particles as a filler material. The laser beam absorption film 2 comprises a gold thin film having a thickness of 300 angstroms wherein 300 angstroms is the absorption wavelength of Ar laser beam so that almost all of the Ar laser beam is absorbed by the gold thin absorption film having the 300 angstroms thickness except for a reflected part of the Ar laser beam by a surface of the gold film. The gold thin film 2 as the laser beam absorption film may be formed on the Teflon film 1 by sputtering. With reference to FIG. 4B, An Ar laser beam radiation apparatus is used. The laser beam radiation apparatus comprises an optical scanner for scanning of the Ar laser beam and a movable stage on which the Teflon film is placed. The movable stage may move both in X and Y directions both os which are vertical to each other. The laser beam is generated by a laser source to be transmitted through a variable attenuator, an ultra sonic tuner, an optical scanner and a condenser lens so that a laser beam 3 having a laser intensity of 30 kW/cm 2 and a beam diameter of 8 micrometers defined by a value of 1/e 2 of beam intensity is irradiated on a surface of the gold laser beam absorption film 2. The optical scanner may show a scanning in an area of 5 cm X 5 cm. The movable stage may permit an enlargement of the laser beam scanning area. At each point, the Ar laser beam is irradiated for 1 millisecond. The irradiation of the laser beam 3 may cause a transformation layer 4 in the Teflon film 1. By use of the above apparatus, the Teflon film is divided into 100 of 5cm-squares for 100 seconds only including alignment processes. With reference to FIG. 4C, the Teflon film is immersed into a water in an ultra sonic generator with an output of 100 W and a frequency 100 kHz so subsequent ultra sonic cleaning for 5 minutes. Ultra sonic wave 5 is propagated through the water in the ultra sonic generator onto the Teflon film 1 with the transformation layer 4. As a result, the transformation layer 4 is dispersed into the water to cur the film 1. The transformed layer has a width of 20 micrometers and a depth of 20 micrometers. The cut section 16 of the Teflon film 1 has a fine vertical edge free from any flash or any curvature as well as none of the transformed layer 4 remains on the cut edge of the film. It is available to vary the time for the laser beam radiation. If the intensity of the laser beam is increased up to 100 kW/cm 2 , then the necessary time for the laser beam radiation at each point is reduced to 10 microseconds. If the intensity of the laser beam is reduced down to 10 kW/cm 2 , then the necessary time for the laser beam radiation at each point is increased up to 100 milliseconds. The intensity and the time for the laser beam radiation are variable to match the conditions of the thickness of the film and the required throughput. The ultra sonic cleaning process may be carried out to subject a plurality of the Tefrlon films 1 with the transformed layers 4 to the ultra sonic wave 5 for simultaneous processes of the plural Teflon films 1 to improve the throughput. Although in the foregoing example the laser beam absorption film 2 comprises the gold film, other materials having a high absorptivity to the laser beam such as metal films, insulation films and semiconductor films as well as carbon based film are available. As the metal film, a titanium film, a nickel film and a copper film are available. As the insulation film, a titanium nitride film is available. As the semiconductor film, an alpha-silicon film is available. A vacuum evaporation method, a sputtering method and a chemical vapor deposition method are also available to deposit the laser beam absorption film on the Teflon film 1. According to the invention, in place of the deposition of the absorption film, it is provided that the Teflon film is exposed to an etchant of sodium naphthalene to thereby form an absorption layer mainly comprising carbon and having a thickness of 0.1 micrometers. The absorption layer has a brown color. In place of the Ar laser beam, Kr laser beam, Nd:YAG laser beam and second higher harmonic waves are available. Further, semiconductor lasers such as near infrared ray or visible rays are available. The use of the semiconductor laser may provide advantages in a reduced cost, a small scale and a facilitation of tuning of a laser output. This advantages may further permit use of a plurality of the semiconductor lasers so that the cutting of the film is obtained by a one time laser beam radiation to improve the throughput. A fourth illustrative example that does not fall within the scope of the present invention will be described with reference to FIGS. 6A to 6B in which another method for cutting an organic insulation film having a relatively large absorptivity to a laser beam is shown. The laser beam is irradiated directly on the organic insulation film without through any laser beam absorption film. The organic insulation film 7 includes a base material of a photosensitive epoxy resin and a filler material of a mixture of calcium carbonate, barium sulfate and glass particles. The organic insulation film 7 has a thickness of 70 micrometers. An Ar laser having a wavelength of 515 nanometers is used wherein ab absorption thickness is approximately 50 micrometers at which the intensity of the laser beam of 515 nanometers wavelength is reduced to 1/e. As illustrated in FIG. 6A, to form the transformed layer 4 in the insulation film 7, a laser beam radiation is carried out at a radiation power of 1W for 6 milliseconds and at a beam diameter of 6 micrometers. As illustrated in FIG. 6B, the ultra sonic cleaning process for the insulation film 7 with the transformed layer 4 is carried out at the same conditions as the third embodiment to thereby the insulation film 7 is cut wherein the cut section 16 has a cutting width of 20 micrometers and a depth of 70 micrometers as well as a fine vertical edge. The irradiation of the laser beam having a wavelength of the same as or near to the absorption wavelength of the insulation film may provide an advantage in a facility of cutting of the film with a large aspect ratio or the cutting section 16 having a large depth as compared to the cutting width. The increase of the cutting width of the cutting section 16 may be obtained by increase of any of the intensity of the laser beam, the time for the laser beam radiation and the laser beam diameter. For irradiation of the laser beam having a wavelength of the same as or near to the absorption wavelength of the insulation film to cut the film with the large aspect ratio, it is available to vary the laser beam wavelength, the base material of the film or to add the photosensitive material or a filler material with a high absorptivity to the laser beam. In place of the Ar laser beam, Kr laser beam, Nd:YAG laser beam and second higher harmonic waves are available. Further, semiconductor lasers such as near infrared ray or visible rays are available. The use of the semiconductor laser may provide advantages in a reduced cost, a small scale and a facilitation of tuning of a laser output. This advantages may further permit use of a plurality of the semiconductor lasers so that the organic insulation film is cut in a one time laser beam radiation to improve the throughput. Whereas modifications of the present invention will no doubt be apparent to a person having ordinary skill in the art to which the invention pertains, it is to be understood that embodiments shown and described by way of illustrations are by no means intended to be considered in a limiting sense. Accordingly, it is to be intended to cover by claims all modifications of the present invention which fall within the scope of the invention as defined by the claims.

Claims: A method of forming via holes (6) in an organic insulation film (1) which comprises a base material of a Teflon resin added with a filler material of glass particles or compound particles, said method comprising the steps of: forming a layer (2) having a high absorptivity to a laser beam on said film (1); exposing predetermined parts of said film (1) to a laser beam to raise the temperature of said exposed parts of said film (1) until said exposed parts are transformed or decomposed; and subjecting said film (1) to an ultrasonic wave (5) so that said transformed or decomposed parts are dispersed to form via holes; wherein, in said step of forming, said layer (2) is formed by subjecting said film (1) to a liquid including sodium naphthalene. The method as claimed in claim 1, characterized in that said laser beam has an absorption wavelength of said film (1). A method of cutting an organic insulation film (1) which comprises a base material of a Teflon resin added with a filler material of glass particles or compound particles, said method comprising the steps of: forming a layer (2) having a high absorptivity to a laser beam on said film (1); exposing predetermined parts of said film (1) to a laser beam to raise the temperature of said exposed parts of said film (1) until said exposed parts are transformed or decomposed; and subjecting said film (1) to an ultrasonic wave (5) so that said transformed or decomposed parts are dispersed to cut said film; wherein in said step of forming, said layer (2) is formed by subjecting said film (1) to a liquid including sodium naphthalene. The method as claimed in claim 3, characterized in that said laser beam has an absortion wavelength of said film (1).

Inventors: BELGACEM HABA, MORISHIGE YUKIO, BELGACEM, HABA, MORISHIGE, YUKIO, BELGACEM, HABA, C/O NEC CORPORATION, MORISHIGE, YUKIO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, H05K3, B23K26